Examples of noxious acidic compounds are HCl, NOx, SO2, SO3.
SO3, for example, is a noxious gas that is produced from the combustion of sulfur-containing fuel. When present in flue gas, the SO3 can form an acid mist that condenses in electrostatic precipitators, ducts or bag houses, causing corrosion. SO3 at concentrations as low as 5-10 ppm in exhaust gas can also result in white, blue, purple, or black plumes from the cooling of the hot stack gas in the cooler air in the atmosphere.
Sodium bicarbonate in powder form is a known basic sodium reagent for cleaning gases of acidic compounds. It finds application in particular for cleaning fumes of oxides of sulphur, of oxides of nitrogen (especially of nitric oxide) and of hydrogen halides of general formula HX (in particular, of hydrogen fluoride and of hydrogen chloride). Fumes of this kind are commonly generated by the incineration of domestic refuse or hospital waste and by the combustion of fuels of fossil origin, especially in electricity-producing power stations.
In these applications, the gas to be cleaned is contacted with the sodium bicarbonate in the form of a finely ground powder at a temperature which is generally between 120 and 250° C. After reaction of the reagent, the flue gas is submitted to a separation in order to separate the reacted reagent from the cleaned flue gas. Separation is frequently operated by means of baghouse filters or electrostatic separators. A flue gas cleaning process based on dry injection of a finely ground powder of sodium bicarbonate is disclosed in EP0740577 (SOLVAY SA).
Sodium sesquicarbonate in powder form is another known basic sodium reagent for cleaning gases of acidic compounds. Sodium sesquicarbonate is commonly used in the form on trona. Trona is a mineral that contains about 85-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. Flue gas cleaning processes based on dry injection of a finely ground powder of trona and subsequent separation of reacted trona are disclosed for instance in U.S. Pat. No. 7,854,911 and U.S. Pat. No. 7,481,987 (SOLVAY CHEMICALS, Inc.).
In those known flue gas cleaning processes, the temperature of the flue gas wherein the basic sodium reagent is introduced is advantageously above critical values, in order for the sodium reagent to have optimal reactivity.
In many situations however, injection into flue gases of very high temperatures creates difficulties during the separation step of the reacted reagents, in view of the required higher thermal resistance of the separators.
US2011/0014106 discloses a flue gas treatment process wherein sodium bicarbonate is pre-calcined, preferably in a fluid bed, before injection in the flue gas.
In IT1306648 is disclosed a process for the cleaning of a flue gas having a temperature below 140° C., wherein sodium bicarbonate is submitted to a thermal treatment in order to decompose it into sodium carbonate, which is thereafter injected into the flue gas.
Those processes have however several disadvantages. The thermal treatment consumes energy and/or requires a separate equipment. Moreover, the handling of the sodium carbonate produced in the thermal treatment is difficult in order to avoid a severe loss of performance.